Method for molding cosmetic composite panels with visible carbon fiber weaves

ABSTRACT

One embodiment of the invention provides a method for compression molding cosmetic panels with visible carbon fiber weaves using clear or lightly filled resins. The method uses a modified, two-step compression molding process to reflow the surface of a partially cured preform of carbon fiber weave and epoxy resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/735,749 filed Apr. 16, 2007, which claims the benefit of U.S.Provisional Application Ser. No. 60/801,256 filed May 18, 2006.

TECHNICAL FIELD

This invention relates to methods for molding cosmetic composite panelswith visible carbon fiber weaves.

BACKGROUND OF THE INVENTION

Composite panels with visible carbon fiber weaves have been molded bythe autoclave cure of a hand lay-up of carbon fiber/epoxy prepreg into asingle-sided mold. After subassembly, if needed, the panels are coatedwith clear primers and/or clear topcoats to satisfy automotive surfacefinishing requirements while maintaining the visibility of the fiberweaves. Resin transfer molding using matched molds and dry fiber weavesis also a known method for molding such composite panels.

SUMMARY OF THE INVENTION

One embodiment of the invention includes a method for compressionmolding cosmetic panels with visible carbon fiber weaves using clear orlightly filled resins. The method comprises using a modified, two-stepcompression molding method to reflow the surface of a partially curedpreform of carbon fiber weave and epoxy resin.

Other exemplary embodiments of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whiledisclosing exemplary embodiments of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 illustrates a method of compression molding a fiber mat and acurable clear resin according to one embodiment of the invention;

FIG. 2 illustrates a product including molded composite panels accordingto one embodiment of the invention.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

The following description of the embodiment(s) is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses.

One embodiment of the invention includes a method of compression moldingcosmetic panels with visible carbon fiber weaves using clear or lightlyfilled resins. As shown in FIG. 1, the method comprises using a two-stepcompression molding method to reflow the surface of a partially curedpreform of carbon fiber weave and epoxy resin. The first step includespreforming compression and the second step includes final molding.

One embodiment of the invention includes placing a fiber mat 10 and acurable resin in a first cavity of a first compression molding tool 20a, 20 b. In illustrative embodiments of the invention, the fiber mat maybe made from at least one of carbon fibers, glass fibers, or othersynthetic or natural fibers. The fiber mat may be woven or unwoven. Thefiber mat may be a carbon fiber weave, for example, TC411 2×2 Twill,available from Sigmatex High Technology Fabrics, Inc., Benicia, Calif. Afirst heat and a first pressure are applied to the fiber mat and thecurable resin to partially cure the resin to a semisolid state toprovide a preform. For example, about 10 to about 60 percent of thereactive groups of the curable resin may cure under the first heat andfirst pressure. In one embodiment, 25 percent of the curable resin maycure under the first heat and first pressure. The first pressure mayrange from about 0.2 MPa to about 0.5 MPa. The applying of the firstheat may result in the material being exposed to a temperature rangingfrom about 80° C. to about 110° C. The curable resin may be cured at thefirst heat and first pressure for about 5 minutes to about 30 minutes.

Next, the preform 30 is placed in a second cavity of a secondcompression molding tool 40 a, 40 b. A second heat and a second pressureare applied to the preform to reflow at least a portion of the curableresin and to cure any uncured portion of the curable resin to form acomposite panel. The curable resin may be cured at the second heat andsecond pressure for about 5 minutes to about 60 minutes. The compositepanel has a substantially transparent resin layer over the fiber mat,and at least portions of the fiber mat are visible through the resinlayer. The composite panel may be substantially free of fiber readout.The thickness of the composite panel may range from about 1.0 mm toabout 3.0 mM.

The second pressure is higher than the first pressure. In oneembodiment, the second pressure is significantly higher than the firstpressure. The second pressure may range from about 1.0 MPa to about 2.0MPa. The applying of the second heat may result in the fiber mat and thecurable resin being exposed to a temperature range of about 130° C. toabout 160° C.

In one embodiment, the second cavity is thinner than the first cavity.This adjustment in the thickness may be accomplished in many differentways. In one embodiment, the thickness adjustment may be accomplished byusing a separate lower cost tool for the preform. The first compressionmolding tool may be a lower cost tool and the second compression moldingtool may be a separate higher cost tool with tight dimensionalprecision. The first compression molding tool may be a soft tool of lessprecise surface definition than the second compression molding tool. Forexample, the first compression molding tool 20 a, 20 b may be a softtool made from relatively soft material such as glass or carbonfiber-epoxy laminates and zinc alloys. The second compression moldingtool 40 a, 40 b may have a high quality surface for the finish molding.

In another embodiment, the first compression molding tool and the secondcompression molding tool may be the same tool. If the same tool is usedfor both steps, then the mold may be designed to close to two differentgaps. The first cavity may close to a first gap and the second cavitymay close to a second gap. In another embodiment, spacers are used inthe cavity of the compression molding tool to create two differentthicknesses. A first spacer may be placed in the first cavity to createa first gap. A second spacer may be placed in the second cavity tocreate a second gap. The first gap may be larger than the second gap.

Another embodiment of the invention uses a collapsing mold to create twodifferent thicknesses. At the preforming stage, a series of air orhydraulic cylinders may hold the mold open the extra thickness neededfor the preform molding. The cylinders may hold the compression moldingtool open the distance of a first gap. Then for the final part molding,the pressure may be released on the cylinders and the mold may close tothe design intent thickness for the part.

In another embodiment, the compression molding tool is built to thepreform thickness and then a disposable shim of plastic film or adisposable film of aluminum sheeting is used to compress the preform inthe second cavity for the final molding.

In another embodiment, the fiber mat may be impregnated with the curableresin before the fiber mat and curable resin (the resin impregnatedfiber mat) are placed in the first cavity of the first compressionmolding tool.

In one embodiment, the curable resin may include an epoxy comprising atleast one diglycidyl ether of bisphenol-A resin. For example, thecurable resin may comprise DER 383 Epoxy Resin, available from Dow. Invarious embodiments of the invention, the curable resin compositionsinclude an epoxy resin which may have an average molecular weightranging from about 300 to about 1200, may have an average number ofrepeating units ranging from about 0.1 to about 2.5, and/or may bepresent in an amount of about 30 to about 80 weight percent.

The curable resin may further comprise a hardener, for example ananhydride. The anhydride may be methyl tetrahydrophthalic anhydride 600,a curing agent available from Lonza Group. The anhydride may also bemethyl hexahydrophthalic anhydride.

The curable resin may further include a filler. In one embodiment of theinvention the filler may include nanoparticles, such as silica ortitanium dioxide based nanoparticles. In one embodiment the filler mayinclude a nanoparticle dispersion, for example silica nanoparticles in adiglycidyl ether of bisphenol-A epoxy resin. In one embodiment of theinvention, the nanoparticles in the dispersion may range from about 10to about 45 weight percent. In one embodiment of the invention, thenanoparticles may be present in an amount of about 0 to about 30 weightpercent. In one embodiment, the silica particles may be present in anamount of about 40 weight percent of a diglycidyl ether of bishphenol-Aepoxy based dispersion, wherein the silica particles have a diameterrange of about 5 nanometers to about 35 nanometers. One example of asuitable nanoparticle dispersion is Nanopox F400, available from HanseChemie AG. Epoxy compositions containing Nanopox F400 may be transparentbecause the nano-sized particles are too small to scatter the visiblelight. The resulting composite panels may be visible carbon fibercomposite panels with further reduced fiber readout since silicaparticles can be used to reduce cure shrinkage and coefficient ofthermal expansion (CTE) of the resin mixtures.

In another embodiment, the nanoparticle filler may comprise a solventbased nano dispersion, for example nanoparticles in methyl ethyl ketone,or methyl alcohol, or isopropyl alcohol. The use of a solvent basednanoparticle dispersion in an epoxy formulation, however, may require anextra procedure to remove the solvent, which would be a contaminant inthe final composite material, in order to obtain a composite withdesired mechanical and thermal properties.

In another embodiment, the curable resin may further comprise acatalyst. The catalyst may be an amine based catalyst. For example, thecatalyst may comprise N,N-Benzyldimethylamine (BDMA), available fromAldrich. In one embodiment the catalyst may be present in amount ofabout 0.4 to about 2.0 weight percent.

To demonstrate embodiments of the invention, carbon fiber compositepanels were prepared using materials and methods described in thefollowing examples.

Material Preparation and Testing

Ingredients used in the examples are summarized in the following table:

Composition Composition II (Parts) III (Parts) Composition I (15 wt %(26 wt % Component (Parts) Nano Silica) Nano Silica) DER 383 Epoxy Resin100 100 0 (Dow) Methyl Tetrahydrophthalic 80 159 80 Anhydride 600 CuringAgent (Lonza AG) N,N-Benzyldimethylamine 2 4 2 (BDMA) Catalyst (Aldrich)Nanopox F400 Dispersion 0 162 167 (Hanse Chemie AG)

Material Performance Example 1

The first example is the control procedure of conventional compressionmolding. Two pieces of carbon fiber mat (TC411 2×2 Twill, T700S, 12Kcarbon fiber, 385 gsm FAW, available from Sigmatex High TechnologyFabrics, Inc., Benicia, Calif.) were cut to size (254 mm×254 mm) andstacked up to form the preform. The preform was saturated with the epoxyresin mixture Composition I to form a resin impregnated preform. Theresin impregnated preform was placed in the preheated mold (254 mm×254mm×1.1 mm) to cure. The resin may be added to the preform either insideor outside of the mold. The resin in this example was cured at 140° C.for 20 minutes under 1.5 MPa pressure to form a 1.1 mm thick visiblecarbon fiber panel.

Example 2

The second example is the new two-step compression molding method. Instep 1 of this method, two pieces of dry carbon fiber weave (TC411 2×2Twill) were placed into the preform mold (254 mm×254 mm×1.1 mm). Thepreform mold may be a soft tool of less precise surface definition thanthe mold in Example 1. The same resin, Composition I, was distributedthrough the fiber preform under 0.4 MPa pressure. The resin was thencured at 100° C. for 10 minutes to form a 1.1 mm thick, partially cured(approximately 25%) preform. At step 2, the partially cured preform wastransferred to the final cure mold (254 mm×254×1.0 mm, with a highquality surface) for the finish molding at the same conditions as usedin Example 1 (140° C. for 20 minutes under 1.5 MPa pressure). During thecure molding in the finish tooling, the surface reflows to give a 1.0 mmthick visible carbon fiber composite panel with an improved surfacequality.

Example 3

In the third example, the two-step compression molding method describedabove in Example 2 was used with two epoxy resin compositions filledwith 15 wt % nano silica (Composition II) and 26 wt % nano silica(Composition III). The source of the nano silica was Nanopox F400, adiglycidyl ether of bisphenol-A epoxy based dispersion consisting of 40wt % of 5-35 nm diameter silica particles. The silica particles mayreduce cure shrinkage and coefficient of thermal expansion (CTE) of theresin mixtures, as shown in the following table. The cure shrinkage (vol%) of the resin compositions was determined by comparing the measureddensities of liquid (before curing) and solid (after curing) resins.

Cure Shrinkage CTE Composition (vol %) (/° C. × 10⁻⁶) I 2.0 60 II 0.9 55III 0.6 50

The two nano filled epoxy compositions were molded into 1.0 mm thickvisible carbon fiber panels using the same carbon fiber weaves and thetwo-step compression molding procedures and conditions as described inExample 2 above.

The levels of fiber readout of the resulting cured composite panels fromExamples 1-3 were evaluated by the Wyko surface profilometer, anon-contact, optical method to measure surface roughness, Ra. Theresults are shown in the following table. Since these panels were madewith a transparent resin, the surface was coated with a sputtered 0.05μm layer of gold to give a reflective surface for the instrument tomeasure.

Example 1/ Example 2/ Example 3/ Example 3/ Composition CompositionComposition Composition I I II III Ra (μm) 1.04 0.86 0.84 0.68

The two-step compression molding method can effectively reduce the fiberreadout of visible carbon fiber composite panels as indicated by thesmaller Ra values measured. The reduction becomes even more significantwhen the method is combined with nano filled epoxy resins.

It should be noted that the compositions disclosed are representative.The compositions are expected to work as well or better within a rangeof the concentration of each component.

Referring now to FIG. 2, in one embodiment of the invention, moldedcomposite panels, as described above, may be utilized to form bodypanels of a vehicle 70 including, but not limited to, the front fascia72, hood 74, front fender 76, front door 78, rear doors (if present),rear fenders 82, trunk lid 84, roof 88, and pillars 90 and 94. Themolded composite panels may be attached to the vehicle body (not shown)which is attached to a vehicle frame (not shown) in a manner known tothose skilled in the art.

Other suitable compositions include those disclosed in Assignee'sco-pending application entitled “Molding Cosmetic Composite Panels WithVisible Fibers From Ultraviolet Light Resistant Epoxy Compositions”(Ser. No. 11/735,718).

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

What is claimed is:
 1. A method comprising: placing a fiber mat and acurable resin in a first cavity of a first compression molding tool;applying a first heat and a first pressure to the fiber mat and thecurable resin to partially cure the resin to a semisolid state toprovide a preform; placing the preform in a second cavity of a secondcompression molding tool; and applying a second heat and a secondpressure to the preform to reflow at least a portion of the curableresin and to cure any uncured portion of the curable resin to form acomposite panel having a substantially transparent resin layer over thefiber mat, and wherein at least portions of the fiber mat are visiblethrough the resin layer, and wherein the second pressure is higher thanthe first pressure, and wherein the applying a second heat and a secondpressure is conducted so that the composite panel is substantially freeof readout, and further comprising placing a first spacer in the firstcavity to create a first gap and placing a second spacer in the secondcavity to create a second gap, and wherein the first gap is larger thanthe second gap.
 2. A method comprising: placing a fiber mat and acurable resin in a first cavity of a first compression molding tool;applying a first heat and a first pressure to the fiber mat and thecurable resin to partially cure the resin to a semisolid state toprovide a preform; placing the preform in a second cavity of a secondcompression molding tool; and applying a second heat and a secondpressure to the preform to reflow at least a portion of the curableresin and to cure any uncured portion of the curable resin to form acomposite panel having a substantially transparent resin layer over thefiber mat, and wherein at least portions of the fiber mat are visiblethrough the resin layer, and wherein the second pressure is higher thanthe first pressure, and wherein the applying a second heat and a secondpressure is conducted so that the composite panel is substantially freeof readout, and further comprising using one of a disposable shim ofplastic film or a disposable shim of aluminum sheeting to compress thepreform in the second cavity of the second compression molding tool. 3.A method comprising: placing a fiber mat and a curable resin in a firstcavity of a first compression molding tool; applying a first heat and afirst pressure to the fiber mat and the curable resin to partially curethe resin to a semisolid state to provide a preform; placing the preformin a second cavity of a second compression molding tool; and applying asecond heat and a second pressure to the preform to reflow at least aportion of the curable resin and to cure any uncured portion of thecurable resin to form a composite panel having a substantiallytransparent resin layer over the fiber mat, and wherein at leastportions of the fiber mat are visible through the resin layer, andwherein the second pressure is higher than the first pressure, andwherein the applying a second heat and a second pressure is conducted sothat the composite panel is substantially free of readout, and whereinthe curable resin further comprises a nanoparticle filler, and whereinthe nanoparticle filler comprises a diglycidyl ether of bisphenol-Aepoxy based dispersion comprising about 40 weight percent of silicaparticles, wherein the silica particles have a diameter range of about 5nanometers to about 35 nanometers.
 4. A method as set forth in claim 3wherein the epoxy has an average molecular weight ranging from about 300to about
 1200. 5. A method as set forth in claim 3 wherein the epoxy hasan average number of repeating units ranging from about 0.1 to about2.5.
 6. A method as set forth in claim 3 wherein the epoxy is about 30to about 80 weight percent of the curable resin.
 7. A method as setforth in claim 3 wherein the curable resin further comprises a hardener.